Thermometer problems - need advice

I just completed a mash of a Vienna lager. Single infusion target was 155, which I hit. However, after the mash I noticed that my thermometer was reading ambient 100 df when the actual temp is only 93. It appears that due to a faulty thermometer and, my negligence in not checking it, my mash temp was only 146-147. Is there something that I can do to fix this? Should I toss the whole thing? I don't want to waste yeast and fermentation chamber space for something that I won't enjoy. Could I do another mini mash with Vienna malt at 158 or so and blend it before pitching?

You'll end up with a drier beer, but it'll still be beer. If you want to bump the final gravity some, you could add some malto-dextrin powder. That would replace unfermentable sugars that mashing low eliminates.

Good suggestion but no malto-dextrin in inventory. I don't know what a dry Vienna would taste like and it definitely isn't what I was looking for. I made a two gallon batch of Vienna and Munich, mashed at 158. Boiling now. I don't know if mixing it will help but I don't suppose it would hurt.

I did the exact same thing with a wit beer. Add malto dextrin IF and only if it ends up being too dry for your tastes. Get a thermopen for your next batch, they are super accurate and usually give a reading of + or - .07 degrees in 3 seconds.

I am definitely in the market for a reliable thermometer. I guess I've been depending on a cheap digital one and that wasn't very smart in hindsight. I'll check out the thermopen. Thanks for the recommendation.

+1 on the Thermapen. Expensive but one of my best purchases so far.

http://www.thermoworks.com/products/thermapen/

There is an open box sale going on for $65

http://www.thermoworks.com/products/thermapen/splashproof_thermapen_open_box_sale.html

Just to follow up. I did mix the two batches, giving me a total of 7.5 gallons into a 6.5 and 3 gallon carboys. After three weeks the gravity was 1.011. Beersmith calculated the FG at 1.014. With the low mash temperature I was afraid that it would come in at 1.005 or less. I've now started dropping the temperature and will soon transfer to kegs for lagering. The hydrometer sample tasted pretty good for a green lager so I have hopes that this will be a good Vienna. It will definitely be beer.

Final followup. I'm finishing the last of the Vienna. It turned out to be almost exactly what I was looking for, a maltier version of Negra Modelo. I'm going to make more soon, hopefully without all of the drama.